phosphotyrosine peptides

I made some petide affinity columns with phosphorylated peptides about
six months ago. Although I have used phosphatase inhibitors in all the
solutions that I have passed over the column, my latest results suggest
that the phosphate group has been removed. Does anybody have any
experience of working with such peptides, and (a) is this normal, or (b)
should I be doing something different when running or storing my
columns? Thanks in advance for your help
Mike Daws